OK, so this has happened in fiction a number of times. It's shown having various effects with Earth wandering through space, getting too close to the sun, whatever. This is not the concern of my friend and I right now.

What would it take to do so? In the fictitious version Earth always appears to just quietly go on its merry way. But I'm asserting that the power needed to do so is such that it would cause the Earth to lose cohesion and fall apart like the asteroid belt. The earthquakes alone would wipe out most life.

So what's the scoop? Is there a way to move a planet out of an orbit without destroying it in the process?

The late Paul Birch of the British Interplanetary Society described one possibility in some detail herehttp://www.orionsarm.com/fm_store/MoveAPlanet.pdf
In short, you send a stream of iron pellets made from asteroids between a ring of electromagnets in orbit around the Earth and another ring around a larger object (usually the Sun) which acts as an anchor. The force used to deflect these pellets will act on the Earth (and to a lesser extent on the anchor object) and gradually move them both with respect to each other.

Birch was primarily concerned with moving Venus, but the same technique could be used on Earth.

Easily. Remember Newton's Laws of Motion. You just need a nearby stellar-mass object close enough and in the right position to negate the gravitational attraction of the sun.

Except remember those pesky forces that create tides in the ocean and cause the crust to flex? Now you've added another. Even the relaxation of stored potential energy in the crust will have dramatic effects on tectonics. Not perhaps world destroying, but enough to create mass earthquakes, tsunamis, and increased volcanism. And any effort to apply force directly to the crust (e.g. a rocket extending outside the atmosphere or some similar direct thrust structure) will cause dramatic effects. The Earth's crust is proportionally much thinner and weaker than an eggshell, and even a thrust structure many tens of miles in diameter would be like a pinhead on the surface.

The Birch proposal outlined by eburacum45 is probably most practical, for generous values of "practical", but realistically the amount of energy required to move the planet from its orbit is vastly beyond anything we can produce today. That's how strong gravity is, and we don't even have to pay a monthly bill for it. Amazing stuff.

Maybe you guys don't appreciate what a knife edge we live on, here on our little planet. Move us a few thousand kilometres nearer or further from the sun and we roast or freeze; alter the ecliptic angle and who knows what would happen to the tide, currents and weather fronts. Speed up or slow down the rotation speed and all that engineering based on one G would mean nothing.

Maybe you guys don't appreciate what a knife edge we live on, here on our little planet. Move us a few thousand kilometres nearer or further from the sun and we roast or freeze; alter the ecliptic angle and who knows what would happen to the tide, currents and weather fronts. Speed up or slow down the rotation speed and all that engineering based on one G would mean nothing.

Well, it's not all that sensitive. The Earth varies in distance to the Sun of a few million kilometer through its not-quite-circular orbit (e~0.016) and has probably bobbled a few degrees off of its current 23.4° inclination to the solar ecliptic throughout the existance of life. (The rotation speed has almost no influence on felt acceleration; we do recalculate weight for large rockets between mid-latitude depot and near-equiatorial launch site, but the resulting difference is generally about a few dozen pounds for a several tens-of-thousands of propellant grain mass motor. On a personal level, you won't notice or care.

However, changes in diurnal rythyms may be problematic for various species, especially intertidal sealife, but that's a lifestyle problem. I think we're better off leaving the Earth as it is and creating our own habitats in space which we can construct and operate on whatever cycles we like, but of course that is too simple a plan.

Probably the biggest effect on a planet with a faster rotation would be more dramatic wind patterns; hurricanes, cyclones and anticyclones would rotate faster, and so on. Having a greater axial tilt would also cause more dramatic and more seasonal weather, although (according to some simulations I've seen) a planet with a greater tilt would probably have smaller ice caps or none at all. None of this would make life impossible, as far as I can see.

Well, it's not all that sensitive. The Earth varies in distance to the Sun of a few million kilometer through its not-quite-circular orbit

Variation in the orbit doesn't necessarily indicate lack of sensitivity

What matters is probably not the momentary condition, but the average of all the various states. Sure, the distance between the earth and sun varies throughout the year, but if you move the earth so that on average, it's a few million kilometers closer or further away than the current average, we might run into trouble.

Another related option that has been studied is the possibility of using a large asteroid in a gravity assist maneuver to transfer orbital energy from the outer planets to Earth in order to expand Earth's orbit.

Estimates are that it would take thousands of gravity assists to make a difference. And a screw-up that results in the asteroid impacting the Earth would be catastrophic, to say the least. But if the alternative is the oceans' boiling away as the Sun expands, we may have no choice but to try it, assuming we develop the capability.

Variation in the orbit doesn't necessarily indicate lack of sensitivity

What matters is probably not the momentary condition, but the average of all the various states. Sure, the distance between the earth and sun varies throughout the year, but if you move the earth so that on average, it's a few million kilometers closer or further away than the current average, we might run into trouble.

"...a few thousand kilometres nearer or further from the sun" is not going to significantly change the thermodynamic state of the Earth, and even a few million is not going to make the planet uninhabitable. The sun goes through phases in variation of irradiance and temperature.

There's a vast gulf between "catastrophic" and "uninhabitable". If global warming were to kill off, say, 5% of the humans on the planet, we'd consider that abominable, but by most species' standards, 95% survival is thriving.

Another related option that has been studied is the possibility of using a large asteroid in a gravity assist maneuver to transfer orbital energy from the outer planets to Earth in order to expand Earth's orbit.

Estimates are that it would take thousands of gravity assists to make a difference.

This is the easiest answer - most of the energy comes from the large outer planets, a tiny fraction comes from maneuvering thrusters that align the trajectories and make it work right.

Assuming the goal is to gently raise the orbit of earth to counterbalance the rising output of an aging sun, an increase of average orbital radius of a million miles over a hundred or thousand years is going to be enough to do the job, and do it very gently in terms of tidal affects on the surface.

I think a major challenge would be to do it without messing up the moon's orbit.

As long at we have massive rocket-powered asteroids transferring all that orbital energy around, it would be difficult to not effect the other planets, so might as well make use of it all. It would be fun to grab Mercury, Venus, and Mars, and put them in orbits at earth's L-points.

It would be nice to move the other rocky planets because it would be a lot easier to terraform them if they were in our Goldilocks orbit and had the same solar input.

"...a few thousand kilometres nearer or further from the sun" is not going to significantly change the thermodynamic state of the Earth, and even a few million is not going to make the planet uninhabitable. The sun goes through phases in variation of irradiance and temperature.

The cheapest way is to boost the earths mass up 12 km/second pro-grade (directly in the line of the orbit - actually instantaneous pro-grade.. as you apply the acceleration you keep it in the actual direction of movement. ) .. Its more expensive to do it by applying the force in some other direction than pro-grade...

Your other question sure reduce the mass of the sun to make the escape velocity (kinetic energy) lesser.. changes the orbit of the earth too

When the Sun leaves the Main sequence it will star losing mass comparatively rapidly, due to a vastly increased solar wind. Some estimates say that this loss will be enough to move the Earth's orbit out beyond the edge of the Sun's red giant phase; others aren't so sure.http://www.astronomy.ohio-state.edu/.../vistas97.html

When the Sun leaves the Main sequence it will star losing mass comparatively rapidly, due to a vastly increased solar wind. Some estimates say that this loss will be enough to move the Earth's orbit out beyond the edge of the Sun's red giant phase; others aren't so sure.http://www.astronomy.ohio-state.edu/.../vistas97.html

By that stage the Earth will have lost its magnetic field due to cooling of the core, so it may no longer have an atmosphere.

The escape velocity from the Sun at the vicinity of the Earth is about 42 kilometers per second. You need to subtract the Earth's orbital velocity from that (assuming you push in the direction the Earth is already travelling to save energy). So the energy needed to accelerate the Earth up to escape velocity from the Sun is:

4.457×1032J

(joules) - see (Wolfram Alpha calculation)

To understand this huge number, let's convert it to some other forms:

It is 13.4 days

of the Total Solar Energy output - see (Wolfram Alpha calculation)
and is the energy obtained if you convert 5×1015 kg
(kilograms) of mass to energy - see (Wolfram Alpha calculation)
To understand this huge mass, it is equivalent to the mass of the all the humans on the planet Earth times 10,000
- see (Wolfram Alpha calculation)

I suppose if you were working very long time scales and could engineer some sort of mass that would stay at a constant distance from the Earth you could, eventually, shift the orbit outward...but we'd probably be talking millions of years (or 10's of millions of years), and today there is no practical way to even start working on something like that....especially if we were wanting to live through the experience.

Note that you wouldn't have to accelerate the Earth to escape velocity in order to move it outwards by a few miilion kilometres. Although the energy required would still be vast.

According to current theories the ice ages on our planet are significantly affected by minor changes in the eccentricity of our orbit; changing the shape of our orbit artificially would certainly cause some very significant changes in our climate.

2. Would the sun losing mass lower its gravity and therefore slip Earth into a wide orbit in the first place?

This is happening right now, but the rate of solar mass loss is so low that it's pretty negligable. AIUI, the Earth is currently moving away from the Sun about 15 cm/year, but most of that is due to tidal drag.

This is happening right now, but the rate of solar mass loss is so low that it's pretty negligable. AIUI, the Earth is currently moving away from the Sun about 15 cm/year, but most of that is due to tidal drag.

Not really sure. The annual increase (15 cm/year) was calculated by a couple Russians (Gregoriy A. Krasinsky and Victor A. Brumberg) using (I think) timing of comunications to interplanetary probes. This paper says that it is probably due to tidal drag. I'm going to guess that it mainly shows up in a change in the semi-major axis.

Not really sure. The annual increase (15 cm/year) was calculated by a couple Russians (Gregoriy A. Krasinsky and Victor A. Brumberg) using (I think) timing of comunications to interplanetary probes. This paper says that it is probably due to tidal drag. I'm going to guess that it mainly shows up in a change in the semi-major axis.

Semi-major- does that mean the orbit becomes more elliptical with time?

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